The present invention relates to a lateral-type semiconductor device, and more particularly to a lateral, bipolar transistor.
In general, a bipolar linear IC includes npn transistors and pnp transistors in a state that the npn transistors are mixed with the pnp transistors, to increase the degree of freedom in circuit design and to simplify the circuit construction of the IC. Further, a vertical npn transistor and a lateral pnp transistor are widely used to form a bipolar linear IC. This is because not only the p-emitter region and p-collector region of the pnp transistor but also the p-base region of the npn transistor can be formed in an n-semiconductor substrate by a common process, and thus the number of fabrication steps of the IC can be reduced.
In a lateral pnp transistor, the breakdown voltage of the collector junction is obtained by spreading a depletion layer into a lightly-doped base (that is, a semiconductor foundation region). Thus, in order to prevent the depletion layer from reaching the emitter junction, that is, in order to prevent the generation of punch-through, the distance between an emitter and a collector (that is, the width of the base) is made greater than the width of depletion layer corresponding to the design breakdown-voltage of the collector junction. As a result, the current amplification factor and cut-off frequency of the lateral pnp transistor are inferior to those of a vertical transistor.
In order to solve the above problem of the lateral transistor, various element structures have been proposed.
For example, a pnp transistor is disclosed in a Japanese patent application JP-A-sho 59-127,865. This transistor includes an n-base region which is formed in a main surface of an n-foundation region and is higher in impurity concentration than the foundation region a p-emitter region formed in the n-base region, a first, p-collector region formed in the foundation region and spaced apart from the n-base region, and a second p-collector region which is extended from the first p-collector region to the n-base region so that the n-base region is surrounded by the second p-collector region. The second p-collector region is smaller in impurity concentration than the first p-collector region and the n-base region. According to the above structure, when a low collector voltage is applied, the second collector region acts as the collector of the transistor. When a high collector voltage is applied, a depletion layer spreads into the second collector region, and the first collector region acts as the collector of the transistor. Thus, the current amplification factor, the cut-off frequency and the breakdown voltage are all improved.
In the pnp transistor disclosed in the above-referred Japanese patent application JP-A-sho 59-127,865, a base electrode is not put in direct contact with the n-base region, but is kept in ohmic contact with the exposed surface of an n.sup.+ -region which is formed so that the foundation region is surrounded by the n.sup.+ -region. Hence, a base current flows between an emitter electrode and the base electrode through the following current path: emitter electrode.fwdarw.p-emitter region.fwdarw.n-base region.fwdarw.foundation region.fwdarw.n.sup.+ -region.fwdarw.base electrode. The base resistance of transistor is determined by the current paths of base current. In the pnp transistor disclosed in the above-referred Japanese patent application JP-A-sho 59-127,865, the current path of base current includes the highly-resistive foundation region. Hence, this pnp transistor is far greater in base resistance than a vertical npn transistor, in which a base current flows only in a base region having low resistance. In a case where the pnp transistor large in base resistance is used for forming a linear IC, there arise the following problems. 1 Variations in base-emitter voltage V.sub.BE based upon a manufacturing process are increased. 2 When the npn transistor and the pnp transistor are both used for forming a circuit, the difference in V.sub.BE between the npn transistor and the pnp transistor and the difference in temperature characteristic between these transistors are increased. 3 Thermal noise due to the base resistance is increased, and readily fluctuates. That is, the characteristics of the linear IC are adversely affected by the large base resistance of the pnp transistor used.